The invention is relative to thermally split zirconium silicate in which amorphous silica phase monoclinic zirconium dioxide crystals, with a defined grain distribution and specific surface, are embedded. Further subject matter of the invention is constituted by the production of this thermally split zirconium silicate and by its use.
Zirconium dioxide (ZrO.sub.2) in the form of naturally occurring baddeleyite or of synthetically produced products is an important raw material in the production of zirconium dioxide ceramics and zirconium silicate pigments. Zirconium dioxide can be obtained by means of chemical decomposition with subsequent hydrolysis of the zirconates obtained at first or by means of the thermal splitting of zirconium silicate (ZrSiO.sub.4) with subsequent separation of the silica. The thermal splitting of zirconium silicate takes place at temperatures above approximately 1700.degree. C.--W. C. Butterman et al., Amer. Mineralogist 52 (1967). At approximately 1760.degree. C., ZrSiO.sub.4 begins to decompose into tetragonal, solid ZrO.sub.2 and liquid SiO.sub.2. Above approximately 2400.degree. C., ZrO.sub.2 and SiO.sub.2 form a uniform liquid which solidifies by means of rapid cooling off--the thermally split zirconium silicate obtained in this manner is an amorphous silica phase in which zirconium dioxide crystals are embedded such that they can be separated by flotation methods and/or leaching methods from the SiO.sub.2 phase.
Thermally split zirconium silicate, that is, ZrO.sub.2 crystals in an amorphous SiO.sub.2 matrix, like those which were known in the past and which have in part found entry into the market, can be obtained in various types of arc methods and plasma methods.
In the plasma methods, for example Great Britain Patent No. 1,248,595 and Ceramics, February 1974, p. 30, a curtain of zirconium sand is allowed to trickle through the flame of the plasma burner, during which the zirconium silicate is thermally split and then cooled off. A complete splitting of the ZrSiO.sub.4 requires the use of extremely finely ground zircon powder; a grinding of the zirconium sand is energy-intensive and, in addition, foreign substances from the powder aggregate are entrained. The products obtainable in the plasma method contain ZrO.sub.2 crystals with a diameter of 0.1 to 0.2 .mu.m and a length of many .mu.ms (see Great Britain Patent No. 1,447,276). The crystals obtained by the plasma method exhibit a different morphology than that of the products in accordance with the present invention.
According to another method, zirconium silicate is melted in an arc and allowed to solidify as a block, the melted body subsequently is broken and ground ((see Gmelin's Handbuch der anorganischen Chemie (Gmelin's Handbook of Inorganic Chemistry), Zirconium, volume 42 (1958), p. 56)). The thermally split zirconium silicate produced in this manner contains zirconium dioxide with an average grain size (d.sub.50 value) of approximately 15 to 20 .mu.m and a BET surface of approximately 0.5 m.sup.2 /g.
Alternatively, the melt can also be removed from an arc melting furnace and converted into a spherical product by means of cooling off in air, as shown in German patent 26 35 030. The applicant of the present invention determined that the grain distribution of the zirconium dioxide in a thermally split zirconium silicate produced in this manner results in an average grain diameter--d.sub.50 value, determined by laser diffraction--of over 3 .mu.m and in a specific surface according to BET of approximately 2 m.sup.2 /g. The determination of substance data took place here, as in the other instances, on zirconium dioxide obtained by leaching out the thermally split zirconium silicate with concentrated sodium hydroxide solution to a residual SiO.sub.2 content of below 0.5% by weight.
German Patent No. 21 43 526 and Great Britain Patent No. 1,447,276 teach that thermally split zirconium silicate is suitable as raw material for ceramic pigments based on zirconium silicate, e.g. inclusion pigments (e.g. zirconium-iron-rose) and pigments of the host lattice type (e.g. zirconium-vanadium-blue and zirconium-praseodymium yellow). It turned out that the increased requirements as regards color intensity and in part also the location of the color are no longer met by the previously accessible, thermally split zirconium silicate pigments. In order to increase the economy of the decorative design of ceramic articles and/or to obtain more brilliant color tones, the technical field is extremely interested in producing more color-intensive pigments. A starting point for the attainment of this goal is to find more suitable raw materials as the source for zirconium dioxide.
The present invention therefore is directed at solving the problem of providing novel, thermally split zirconium silicates which are more suitable as a source for zirconium dioxide in the production of the industries' more extracting zirconium silicate based pigments (i.e., thermally split zirconium silicates which permit more color-intensive pigments to be produced than the previously known thermally split zirconium silicates).
Thermally split zirconium silicate produced by the method of the present invention is characterized in that the zirconium dioxide embedded in an amorphous silica phase exhibits an average grain size (d.sub.50 value) in a range of 0.5 .mu.m to 3.0 .mu.m and a specific surface (BET) in a range of 3 to 15 m.sup.2 /g. The zirconium dioxide exhibits, as is apparent from the scanning electron microscope photograph of FIG. 3, a typical morphology which could be designated as dendrite-shaped.
Preferred products of the present invention exhibit d.sub.50 values in a range of 5 m.sup.2 /g to 12 m.sup.2 /g for the zirconium dioxide embedded in the SiO.sub.2 phase. Especially preferred products are distinguished by a very narrow grain spectrum of the zirconium dioxide and illustrated by the fact that at least 90% of the zirconium dioxide consists of particles with a diameter less than 10 .mu.m, especially less than 5 .mu.m and greater than 0.2 .mu.m. The grain distribution, including the d.sub.50 value, was determined by means of laser diffraction with water as the suspension liquid, sodium pyrophosphate as dispersing agent and 5 minutes ultrasound mixing in an HR 850 granulometer of the Cilas-Alcatel company. The BET surface was determined with nitrogen as adsorption gas in accordance with DIN 66131. The material characteristics were determined for zirconium dioxide obtained by leaching with concentrated sodium hydroxide solution.
The thermally split zirconium silicates of the invention differ, as explained above, in their material characteristics from those of previously known products. Previously known thermally split zirconium silicates, like those which have previously gained favor in the production of pigments, exhibit higher d.sub.50 values and, in addition, a lesser BET surface. In contrast to the previously known products, zirconium silicate pigments of the host lattice type and inclusion type can be produced by the novel method of the present invention so as to have greater color intensity and a shifting of the particular color location in the desired direction along the color spectrum. Of the zirconium silicate pigments of the host lattice type, there is known pigments in particular in which the Z.sup.4+ positions in the lattice are occupied in a valently compensated manner by chromophoric ions such as vanadium-(blue), praseodymium-(yellow) or terbium ions (yellow) (see U.S. Pat. No. 2,441,447; Great Britain Patent No. 1,447,276; and prospectus 2/83, No. 59 of the Th. Goldschmidt Company). Of the pigments of the inclusion type, there is known pigments in particular in which the color-bearing component such as e.g. cadmium sulfoselenides, iron oxides, iron titanates, colloidal metals and iron-manganese compounds are inclosed in a casing of zirconium silicate (see German Patent No. 23 13 535; German Patent No. 23 23 770; German Patent No. 21 43 525; German-OS Patent No. 39 06 818; and European Patent No. 0,294,664). The advantageous effect, which could not have been foreseen, which the thermally split zirconium silicates of the present invention exert on the pigments follows from examples 2,3 and reference examples 2,3.
A further use of the products of the invention consists in the obtaining of the monoclinic zirconium dioxide contained in the thermally split zirconium silicate by means of known leaching methods. Among the known leaching methods, the one which uses concentrated alkali lyes, especially 30-50% by weight sodium hydroxide solution, are to be emphasized. The thermally split zirconium silicate is treated to this end once or repeatedly with the lye at in general 100.degree. to 200.degree. C., especially 115.degree. to 170.degree. C. under atmospheric or increased pressure and supplied to a solid-liquid phase separation. The leaching can be carried out isothermally or isobarically. At least 2 moles of alkali hydroxide, relative to one mole of the SiO.sub.2 contained in the thermally split zirconium silicate, are used. The duration of the leaching, which is generally between 1 and 10 hours, is a function of the temperature, the lye concentration and the grain size of the thermally split zirconium silicate used. Zirconium dioxides with an SiO.sub.2 content below 0.5% by weight are obtainable in this manner.
It was found that the thermally split zirconium silicate of the invention can be produced by means of inductively melting zirconium silicate in a high-frequency or medium-frequency induction furnace, for example, a coreless induction furnace, with a sintering crust crucible at a temperature in a range of 2500.degree. to 3000.degree. C. and by a subsequent quenching of the melt. The method is characterized in that the melt is drawn off in the form of a stream, this stream is fanned out in a free fall by blowing on it with an inert, cool gas and/or by spraying it with water and quenching the melt thereby and comminuting the product obtained in this manner as required in a breaking and/or grinding manner.
It is advantageous if a thin molten stream with a width of especially 5-20 mm is drawn off via a channel member extending off from the crucible and if compressed air is blown on it with one or several nozzles. In general, the gas used should be at room temperature but higher or lower temperatures are also possible. Alternatively, the melt is sprayed, also while in free fall, with water from one or several nozzles. According to an especially preferred embodiment, air is first blown on the molten stream and then, while it is still in a free fall, water is sprayed on it, prefereably from two or more optionally adjustable, superposed nozzles. In order to quench the drawn-off melt, an amount of air in the range of 0.1 to 3 Nm.sup.3 air per kg melt is generally sufficient. For the quenching by spraying with water, an amount of water in a range of 10 to 100 l per kg melt has proven to be suitable. The product, which has now solidified, can be further cooled, to the extent necessary, in a water basin or a water groove. The product precipitates in the form of granules with a length of approximately 1 m to 10 mm and can be dried and broken as required in a known manner and/or be ground dry or wet to the desired degree of grinding.
The manner of quenching is decisive for the material characteristics of the thermally split zirconium silicate and of the zirconium dioxide contained in it. A slow cooling off of the melt results in larger ZrO.sub.2 crystals which entail the above-described disadvantages in the production of pigments. A simple pouring of the melt into water yields products with a very broad grain spectrum, which is disadvantageous for the formation of pigments and which is outside of the claimed range.
In order to melt the zirconium silicate for thermal splitting, those furnaces that are potential candidates are those in which a uniform melt can be produced and drawn off in the form of a thin stream. The melting can be effected e.g. by means of an arc or by inductive heating. An induction melting furnace with sintering crust crucible like that known from European Patent No. 0,119,877 is very well suited.
Zirconium silicate is cited by way of example in European Patent No. 0,119,877 as a material to be melted; however, there are no suggestions about the thermal splitting and the manner of how melt should be uniformly removed and quenched. According to an embodiment of European Patent No. 0,119,877, the furnace comprises an optionally cooled tube exiting laterally through the wire coil which tube is intended to serve for the removal of the melt; the melt is allowed to run out into a water basin. In the case of a melt of thermally split zirconium silicate, an increase in volume occurs during the cooling off, so that the flow-off tube becomes clogged: An opening of the run-off tube with chiseling and boring tools proved to be unsatisfactory on account of the hardness and brittleness of the solidified melt.
In order for the method to succeed, it is essential that a uniform stream of melt can be removed from the furnace and supplied to the quenching device. This succeeds if the method is carried in a semi-continuous manner in an induction melting furnace with internal wall provided by a melting inductor coil structure which encases a sintering crust crucible. A part of the melt, preferably 5 to 30% of the crucible contents, is removed at periodic intervals and an appropriate amount (e.g. an amount which when melted equals the amount discharged) of zirconium silicate is supplied to the crucible. An open run-out channel, which is located at the upper edge of the inductor coil structure and is intensively cooled, is used as the run-out device. The melt run-out is started at periodic intervals by broaching the melt with a broaching device and the run-out amount is regulated as required by regulated tipping of the furnace with a tipping device. The broaching device comprises a broaching lance and automatically controllable devices for varying the angle of inclination of the lance or for a vertical parallel shifting of the broaching lance, which is positioned horizontally or at an incline, and for extending and retracting the lance. The broaching lance is guided in such a manner that it first catches below the melt nose of the solidified melt from the preceding melt broaching remaining in the run-out channel and the lance is then raised so as to lift the nose and adjacent solidified material. The lance is subsequently driven forward between the raised, solidified melt and the bottom of the channel until the sintered crust formed in the inlet area of the channel is pierced.